1. Field of the Invention
The present invention relates to an optical pickup apparatus, an optical read/write apparatus, and a gap control method applicable to an information recording medium that uses near-field optics.
2. Description of the Related Art
In recent years, there has been interest in read/write techniques using near-field optics (also referred to as evanescent waves), wherein lights spills out from the interface when the inter-object spacing becomes less than a certain distance. Such near-field techniques are of interest in order to achieve high write densities and high resolutions in information recording media such as optical discs and optical memory cards. Such near-field optical read/write techniques involve extremely fine control, wherein the gap between the lens or other near-field optical radiating means and the surface of the information recording medium is typically on the order of one-fifth to one-half the wavelength of the light used to read and write information.
The focusing optics for generating near-field light may involve a high numerical aperture (NA) objective lens made from an aspherical or similar lens, as well as a solid immersion lens (SIL) interposed between the objective lens and the information recording medium. Using such an SIL involves maintaining the gap between the SIL and the surface of the optical disc or similar information recording medium such that the gap distance generates near-field light (i.e., the gap is not more than one-half to one-fifth the wavelength of the light, as described above). Furthermore, the position of the SIL is also controlled in the above case, in order to track runout of the information recording medium (i.e., disc surface runout in the case of disc-shaped information recording media). For this reason, control methods have been proposed wherein, for example, a desired gap size is detected and maintained using the amount of reflected light due to total internal reflection (see Japanese Unexamined Patent Application Publication No. 2001-76358, for example).
The control method cited above utilizes the fact that the amount of reflected light due to total internal reflection is proportional to gap size at distances whereby near-field light is generated. More specifically, the above method maintains a fixed gap by using a feedback servo loop that stabilizes a servo loop system with a phase compensation filter, with the amount of reflected light due to total internal reflection being used as a gap error signal.
Consider the following example, wherein the distance for generating near-field light is maintained at a reference value of 20 nm, the allowable deviation is 5 nm, the allowable surface runout is 40 μm, and the rotational frequency of the disc-shaped information recording medium is 3000 rpm. Given the above values, the band becomes 8 kHz. In practice, however, disturbances due to disc rotation result in pronounced rotationally synchronous components, and thus it becomes difficult to control the gap size with precision, even when a band greater than 8 kHz is achieved.
In order to resolve this problem, methods have been proposed wherein a repeating servo is used (see Japanese Unexamined Patent Application Publication No. 2006-313589, for example). The repeating servo stores the error signal for a single rotation in external memory. By subsequently applying the gap error signal of an ordinary feedback servo loop as a feed-forward signal, gain having a peak at the rotational frequency component is obtained. (See also T. Ishimoto et al., “Technologies for removability in a near-field optical disc system”, Proceedings of SPIE Reprint, Vol. 6282, 62820C, 2006.)